A conventionally-known ink ribbon used in a thermal printer or the like has a structure having a base sheet such as a polyester film; a heat-melting-type thermal recording layer or a sublimation-type thermal recording layer on one side of the base sheet; and a back layer on the opposite side (a layer contacting with a thermal head). As materials for forming a back layer, for example, a crosslinked and hardened resin has been proposed, which resin comprises a silicone-modified polyurethane resin and polyisocyanate (see JP S61-227087 and JP S64-11888) or with an acryl-silicone graft copolymer (see JP S62-30082, JP H01-214475, and JP H02-274596). The back layer is requisite for properties that, upon the contact of a thermal head on a back layer, cause non-dropping any powder from the back layer by the contact friction; and causes non-sticking to the thermal head on the back layer in addition to appropriate slipperiness, heat resistance, and adhesiveness to the base sheet,
However, the above proposed techniques are not enough ones for satisfying all the back layer's needs; and ink ribbons with a back layer of higher performance have been desirable.
An object of the present invention is, therefore, to solve the above-described problems, and provide an excellent ink ribbon having a back layer that has excellent adhesiveness to the base sheet; excellent heat resistance relative to a thermal head; good printing; no migration of impurities from a back layer into the ink layer; and no sticking to a thermal head.
The inventors of the present invention have proceeded with an extensive investigation to develop an ink ribbon overcoming the above problems; paid an attention to a phenomenon described in “The Relationship Between Phase Structure and Interface Structure Of Segmented Polyurethane Resin Introduced with Functional Groups” (Katsuhiko Nakamae, Seiji Asaoka and Sudaryanto, Journal of the Adhesion Society of Japan, Vol. 31 (No. 3), Pages 70 to 75 (1995)): For example, the phenomenon that a polyurethane resin containing sulfonic groups as side groups therein causes molecular phase separation upon the formation of a dried coating film on a substrate by using the polyurethane resin, and the sulfonic groups are internally oriented towards a surface of the substrate, but not towards externally; the inventors reached to an idea that this phenomenon is applicable to the development of an excellent ink ribbon; the use of a polyurethane resin containing sulfonic acid groups as side groups in its molecule as a material for the back layer of the ink ribbon causes increased adhesiveness of the back layer to a base sheet, because such a polyurethane resin causes molecular phase separation upon the formation of the back layer on the base sheet. As a result, the sulfonic acid groups is oriented forwards the base sheet; and the orientation results in excellent adhesiveness of the back layer to the base sheet. This idea leads to the completion of the present invention.